The challenge of designing a next generation compact Vertical Take Off and Landing (VTOL) craft has been addressed with the combination of a novel radial fan technology and the use of unique lifting and control surfaces.

Above: Using an axis symmetric mesh
provides the best compromise of
quality, density and size

CFD Techniques The design process started with a searching aerodynamic analysis in order to establish the best interaction between the lifting surfaces and to set the parameters of the propulsion system that satisfy the lift requirements. During the concept creation stage, no prototype was built, and the geometry development relied only on the CFD results. To ensure a high turnover of results for each configuration, automation scripts were written to create, mesh and run a matrix of geometries and boundary conditions.

In the next step where the flight mechanics and stability are analyzed, all surfaces relevant to the control of the craft are modeled in detail. However, to reduce the complexity, rotor and stator blades are simulated through a momentum generator, using the user subroutines capability of STAR-CCM+. The radial momentum added to the system converges on the value of the power input needed to hover in each case. An additional swirl can also be added to accurately simulate any residual tangential flow.

The mesh generation and model setup is controlled by a script that implements the CD-adapco automatic meshing feature when running a series of cases at different control surface configurations and flight orientations. The flight control system analysis has proved essential in the optimization of the performance of the attitude control system. For example, the flight performance of the manned platform in particular required detailed analysis of its behavior in ground effect. This flight control system analysis returns accurate aerodynamic forces and pitch, roll and yaw torque inputs to the flight controls system lookup tables, with up to four configurations being run daily on the solving cluster.

Simultaneously, the propulsion system and lifting surfaces are analyzed in greater detail. Sector meshes are set up for the Moving Reference Frame (MFR) method to analyze the propulsion turbo machinery. Special attention is paid to the rotor and stator interactions with the blades optimized to satisfy the dual requirements of efficient lift generation and rotor torque cancellation.

During the same design loop, the yaw control surfaces capabilities can be evaluated in order to complete the range of information needed for the flight controls systems. Mesh size and setup arameters have been optimized to allow at least one configuration to be run overnight. This stage closes the aerodynamic design loop, as shape and dynamic loads are then known for the CAD/FEM team to finalize the model.

Benefits and Achievements The STAR-CCM+ simulation process is fully integrated into the virtual design process and interacts strongly with the CAD design and software development for the control system. The design loads predicted by the CFD analysis make the choice of the composite materials in the craft’s structure much easier, leading to significant weight reductions and further improvements in the payload and endurance capabilities of the flight platform.

The flight control system CFD analysis has proven to be a powerful tool. One of its most important outputs is the data that is fed into a flight simulator that delivers realistic attitude resonse and lift characteristics. It has also made it possible to identify, quantify and address an unusual ground effect response and therefore avoided putting the prototype craftor personnel at risk.

Once the CFD calculations of aerodynamic performance and attitude control met the prerequisite targets, a prototype flight platform was constructed.

The successful test flights of this “MuPod” UAV (unmanned aerial vehicle) has confirmed the value of CD-adapco products in providing accurate flight characteristics early in the development process. It was very rewarding to witness the technology at work as the prototype took off for the first time and behaved as predicted by the flight simulator.

Evolution The virtual design environment approach has provided an early and thorough understanding of the potential and capabilities of this innovative flight technology. The number of hardware variants selected for construction has been reduced significantly by using the right tools and the right techniques and significant savings in time and cost have been realized as a result. We are currently updating the implementation methodology to use STAR-CCM+ with very promising results so far.